Liquid crystals are highly anisotropic fluids that exist between the boundaries of the solid and conventional, isotropic liquid phase. The phase is a result of long-range orientation ordering among constituent molecules that occurs within certain ranges of temperature in melts of many organic compounds. The ordering is sufficient to impart some solidlike properties on the fluid but the forces of attraction usually are not strong enough to prevent flow. Liquid crystallinity also is referred to as mesomorphism. Liquid crystals are in thermodynamic equilibrium over wide temperature ranges and undergo well-defined phase changes. Molecular interactions are known to contribute greatly to the organization of liquid crystalline states since the combination of molecular shape, size, and orientation of its interactions with neighboring species contribute to the liquid crystallinity of a molecule. The general, common molecular feature of such compounds is an elongated, narrow molecular framework, which usually is depicted as a rod or cigar-shaped entity.
The mixing of liquid crystal compounds to lower the solidifying point of liquid crystals and thereby extend the liquid crystal phase is known from numerous sources, including U.S. Pat. Nos. 4,090,975, 4,137,192, and 4,668,426. In each of these cases, the lowering of the melting point is merely an application of Raoult's law and the "stabilization" only results from a depression of the melting point; no intermolecular forces such as hydrogen-bonding are involved.
While it is known that intramolecular hydrogen bonding can induce mesomorphism, c.f. non-mesomorphic 4-amino-4"-nitro -p-terphenyl vs. mesomorphic 3-nitro-4-amino-4"-nitro -p-terphenyl, it is also known that hydrogen bonding may lead to nonlinear molecular associations that disrupt and prevent mesomorphism. Hydrogen-bonding associations also may be so strong that by the time the solid reaches its melting point, the thermal energy is too intense to permit substantial order to remain within the fluid and thus the solid passes directly into the isotropic liquid. (Kirk-Othmer Encyclopedia of Chemical Technology, Third Ed., Vol. 14, 413 (1982))
Several simple para-n-alkoxy substituted benzoic acids are known to form liquid crystalline phases due to dimer formation (G. W. Gray et al., J.Chem.Soc., 4179 (1953)). Also mixtures of other similar dimers have been suggested (S. Takenaka et al., Mol.Cryst.Liq.Cryst., 90, 365-371 (1983)) as also being liquid crystalline. Since the benzoic acids always exist as dimers in both the crystalline state and the mesophase, the dimer per se is art-recognized as a single component mesogen. In the present invention, it is the mixing of two independent and different liquid crystalline components capable of hydrogen bonding to one another that leads to the formation of the new mesogens having the enhanced mesophase stabilization properties as evidenced by an elevated mesophase (or liquid crystalline state) to isotropic transition temperature.
L. J. Yu et al., Mol.Cryst.Liq.Cryst., 54, 1-8 (1979) discloses a mixture of a cyano-substituted biphenyl and a p-pentylbenzoic acid shows induced smectic mesomorphism. The authors have found that the dimeric benzoic acid behaves as an undissociated entity in its interaction with the biphenyl compound, and it is only the dimer that so interacts, but clearly not by hydrogen bonding. Also, there is no elevation of the nematic to isotropic transition temperature that occurs with the new hydrogen bonded mesogens of the present invention.
The abstract of Schroeder et al., J.Org.Chem., 41, 2566 (1976) states "Within the well-ordered parallel molecular alignment of the mesophase, hydrogen bonding is no longer a detriment to mesomorphism and may, in fact, enhance it." While this might appear to impact upon the present invention, a careful reading of the reference clearly shows that the abstract is not indicative of the content of the article. The major finding is actually that some compounds which form hydrogen bonds can exhibit a mesophase, i.e. that hydrogen bonding does not prevent liquid crystallinity as some had thought. This is not the same as finding that hydrogen bonding is responsible for mesomorphism as in the present invention. It should be noted that Schroeder does not even consider mixtures of different compounds, let alone suggest an interaction therebetween.
In view of the stringent requirements of broad mesophase range, chemical stability, viscosity, dielectric and elastic constants, etc. for liquid crystal applications, there continues to be great interest in the synthesis of new liquid crystalline substances, i.e. mesogens, to meet the changing and increasing demands of this technology. The present invention arose from research directed to creating new mesogenic materials having an extended mesophase range and particularly mesophase to isotropic transition temperature which is above that of any of the individual components used to produce the new mesogenic materials.
Accordingly, it is an object of the present invention to develop new mesogenic materials which have increased mesophase to isotropic transition temperatures.
It is a further object to develop new mesogenic materials having increased mesophase stability as compared to any of the components utilized to produce them.
It is a further object to develop a method of producing additional mesogens which method will avoid having to conduct complicated reaction schemes.
These and still further objects will be apparent from the ensuing description of the invention.